[C++] Allow RecordBatch.length to be less than array lengths

[asfimport]

0.13 ignored RecordBatch.length.  0.14 requires that RecordBatch.length and array length be equal.  As per https://lists.apache.org/thread.html/2692dd8fe09c92aa313bded2f4c2d4240b9ef75a8604ec214eb02571@%3Cdev.arrow.apache.org%3E , we discussed changing this so that RecordBatch.length can be [0,array length].

 If RecordBatch.length is less than array length, the reader should ignore the portion of the array(s) beyond RecordBatch.length.  This will allow partially populated batches to be read in scenarios identified in the above discussion.

  Status GetFieldMetadata(int field_index, ArrayData* out) {
    auto nodes = metadata_->nodes();
    // pop off a field
    if (field_index >= static_cast<int>(nodes->size())) {
      return Status::Invalid("Ran out of field metadata, likely malformed");
    }
    const flatbuf::FieldNode* node = nodes->Get(field_index);

*    //out->length = node->length();*
*    out->length = metadata_->length();*
    out->null_count = node->null_count();
    out->offset = 0;
    return Status::OK();
  }

Attached is a test IPC File containing a batch with length 1, array length 3.

Reporter: John Muehlhausen / @jgm-ktg

Original Issue Attachments:

• test.arrow_ipc

PRs and other links:

• GitHub Pull Request #5663

_{Note: This issue was originally created as ARROW-5916. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I think that the fields' length needs to be respected in GetFieldMetadata (since this can vary for nested fields). The appropriate fix is probably here:

https://github.com/apache/arrow/blob/master/cpp/src/arrow/ipc/reader.cc#L368

If the arrays come back from LoadArray longer than num_rows, then the record batch should be constructed then sliced.

[asfimport]

Wes McKinney / @wesm:
We didn't reach a conclusion on this so closing for now